Liquid crystal display (LCD) devices have become increasingly important in displays that require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat surface. For example, LCDs are used in display devices such as wristwatches, pocket and personal computers, flat panel television displays and aircraft cockpit displays. When there is a need to incorporate a color display capability into such display devices, a component called a color filter is used. For the device to have color capability, each LCD pixel is aligned with a color area, typically red, green, or blue, of a color filter array. Depending upon the image to be displayed, one or more of the pixel electrodes is energized during display operation to allow full light, no light, or partial light to be transmitted through the color filter area associated with that pixel. The image perceived by a user is a blend of colors formed by the transmission of light through adjacent color filter areas.
A major contributor to the cost of color LCDs is the color filter. Four color filter manufacturing methods are known in the art, viz., dye gelatin, pigmented photoresist, electrodeposition and printing. The pigmented photoresist method offers the best trade-off of degradation resistance, optical properties, and flexibility along with high resolution, and is generally preferred. While conventional photolithographic materials and methods may be employed in the photoresist method, it suffers from the high cost and inconvenience associated with numerous process steps, some involving wet chemistry.
Laser-induced thermal transfer processes are well-known in applications such as color proofing and lithography and have been described in, for example, Baldock, U.K. Patent 2,083,726; DeBoer, U.S. Pat. No. 4,942,141; Kellogg, U.S. Pat. No. 5,019,549; Evans, U.S. Pat. No. 4,948,776; Foley et al., U.S. Pat. No. 5,156,938; Ellis et al., U.S. Pat. No. 5,171,650; and Koshizuka et al., U.S. Pat. No. 4,643,917.
As is known in the art, laser-induced processes use a laserable assemblage comprising (a) a donor element containing the material to be transferred in contact with (b) a receiver element. The laserable assemblage is exposed to a laser, usually a suitable spatially modulated near-infrared laser, resulting in transfer of material from the donor element to the receiver element. To form an image, exposure takes place over a small region of the laserable assemblage at any one time, so that transfer of material from the donor element to the receiver element can be built up one pixel at a time. In this context the term pixel indicates the minimum addressable writing area of the laser exposure system. This laser addressable pixel size is generally smaller than the LCD color pixel size described above. Computer control of the writing laser produces transfer with high resolution and at high speed. The laserable assemblage, upon imagewise exposure to a laser as described supra, is henceforth termed an imaged laserable assemblage.
For the preparation of images for proofing applications and in photomask fabrication, the colorant comprises a pigment or a dye. For the preparation of lithographic printing plates, the colorant comprises an oleophilic material that will receive and transfer ink in printing.
Laser-induced processes are fast and result in transfer of material with high resolution. However, in many cases, the resulting transferred material does not have the required durability. In dye sublimation processes, light-fastness is frequently lacking. In ablative and melt transfer processes, poor adhesion and/or durability can be a problem. In U.S. Pat. Nos. 5,563,019 and 5,523,192, improved multilayer laserable assemblages and associated processes are disclosed that do afford improved adhesion and/or durability of the transferred images. In U.S. Pat. No. 6,051,318 an improved donor film for use in the production of color filters is disclosed. U.S. Pat. No. 6,143,451 discloses a laser-induced thermal image transfer imaging process characterized by the use of an ejection layer that affords advantages in the final imaged product.
As is known in the art, the thermally imageable layer in a laserable assemblage always contains some sort of binder, generally a polymeric binder. The binder serves to hold together the colorant and any adjuvants thereto before, during and after the image transfer process is effected, forming a single cohesive, homogeneous mass. It is found that the physical properties of the binder have significant effect on the properties of the transferred image. In particular, it has been found in the practice of the art that binders characterized by glass transition temperatures near or below room temperature provide good toughness and durability with superior adhesive properties, but often at the expense of resolution. On the other hand, binders characterized by glass transition temperatures well above room temperature provide superior resolution but at the expense of toughness, durability, and adhesion. Practical application of laser-induced thermal image transfer to high resolution applications such as color filter formation requires toughness and adhesion sufficient to permit survival of the transferred image during the remainder of the manufacturing process. The resolution requirements for the color filter application are extremely demanding, and little trade-off can be made while preserving utility in the application.
Aqueous blends of colloidally dispersed polymers for use in making organic coatings which are hard and ductile at ambient temperature and which remain stiff and elastic at elevated temperature are disclosed in Mazur et al, U.S. Pat. No. 6,020,416. The combination of properties is attributed to the use of blends of high molecular weight polymers differing in glass transition temperature.
A need exists for stable crosslinked pigmented images on a substrate wherein the surface of the image away from the substrate is an extremely smooth surface.